The present invention relates to composite materials, which are based on polyamides as matrix compound and natural fibers. The invention does also include moldings manufactured from such composite materials, which are stable against humidity and elevated temperature.
EP1129139 discloses compositions to be processed in molten form. They shall include organic fibers which are embedded in natural polymers like starch or cellulosic material. These mixtures have some disadvantages in processing, so according to the invention shellac has to be added as processing additive. Parts form such components are not very stable against weathering conditions and humidity.
U.S. Pat. No. 5,317,037 relates to melt-moldable compositions which can be shaped into useful articles which have sufficient strength in a dry environment. The composition contains organic fibers and meltable, water-soluble binders. As binders natural polymers are disclosed like sugar, polydextrose, gelatin, gluten, hydroxymethyl cellulose, gum arabic and starch. Such polymers shall be water soluble with the effect that such compositions rapidly disintegrate and degrade in a wet environment.
WO2008/107615 discloses compositions which contain polyamides, polylactic acid, eventually an agent for better compatibility and natural or synthetic fibers. Polylactic acid as polymer contains many polar groups, so the stability against moisture is low. Also the mechanical properties of such formed parts do not meet many requirements.
WO2008/064950 discloses molded parts made from polymers from the hot melt type. Also specific polyamides are disclosed, but no compositions containing fibers are mentioned. The molded parts shall have the ability to get molten at one end so that they can be glued to metallic surfaces.
WO00/59989 discloses composite materials from natural fibers and as matrix material oleochemical thermoplastic polymers such as polyester, polyamides or polyesteramides. As component dimer fatty acid is listed together with other useful monomeric components. As examples polyamides based on dimer fatty acid/acelaic acid and dimer amines are disclosed. No specific composite compounds or specific polyamides are disclosed.
Polyamides are known in different compositions. According to their chemical structure, they can have softening points in a wide range. A principal problem of such polyamides is the relatively high melting temperature. This is important, as for many applications polyamides are used in an injection molding process. In this process it is advantageous to have a lower melting point, so that the energy costs can be saved. Additionally the viscosity of such polymers will be lower which influences the manufacturing process. On the other hand if the parts manufactured from such polyamides are used in the practice, they should have stable properties even at higher temperature. If the softening point is low the mechanical stability of such parts will decrease, e.g. in the vicinity of heat evolving machines such parts are not usable. So the use of such parts made from polyamides is limited. It is known to use component parts which have to perform a variety of functions composed of two or more layers which have to be bonded to one another at a bonding surface. This requires two different materials, the basic materials are expensive and additionally the manufacturing is difficult.
Fibers of oil based origin are known. But composition and surface often lead to problems in the adhesion to a matrix material. Natural fibers have fewer chemical variances at the surface so they show good adhesion to polar polymers. Such fibers of natural origin may be susceptible to water. This humidity may influence the mechanical properties of the material.